Overcurrent protection in amplifier topologies employing DC isolation

ABSTRACT

An amplifier having output switch circuitry is described. A sense resistor is operable to transmit an output current associated with the output switch circuitry. Low-voltage circuitry is operable to provide a drive signal to the output switch circuitry. Isolation circuitry is operable to provide DC isolation between the low-voltage circuitry and the output switch circuitry. Current sensing circuitry is directly connected to the sense resistor and is operable to sense the output current in the sense resistor and generate a fault signal in response to an overcurrent condition. The fault signal is characterized by a signal level which is compatible with the low-voltage circuitry without additional DC isolation.

RELATED APPLICATION DATA

The present application claims priority under 35 U.S.C. 119(e) to U.S.Provisional Patent Application No. 60/520,904 for OVERCURRENT PROTECTIONIN AMPLIFIER TOPOLOGIES EMPLOYING DC ISOLATION filed Nov. 17, 2003 theentire disclosure of which is incorporated herein by reference for allpurposes.

BACKGROUND OF THE INVENTION

The present invention relates generally to overcurrent protection, andmore specifically to providing overcurrent protection in amplifiertopologies which employ DC isolation.

DC isolation is employed in many amplifier topologies to separaterelatively high power output stages from the relatively low powercircuits which drive them. It is desirable to provide such isolation fora variety of reason, e.g., to reduce fabrication costs, especially incases where the lower power circuits may be characterized by more thanone signal or power supply level. Overcurrent protection schemes whichare compatible with such isolation techniques are also desirable.

SUMMARY OF THE INVENTION

According to the invention, compatible overcurrent protection and DCisolation techniques are provided. According to a specific embodiment,an amplifier having output switch circuitry is provided. A senseresistor is operable to transmit an output current associated with theoutput switch circuitry. Low-voltage circuitry is operable to provide adrive signal to the output switch circuitry. Isolation circuitry isoperable to provide DC isolation between the low-voltage circuitry andthe output switch circuitry. Current sensing circuitry is directlyconnected to the sense resistor and is operable to sense the outputcurrent in the sense resistor and generate a fault signal in response toan overcurrent condition. The fault signal is characterized by a signallevel which is compatible with the low-voltage circuitry withoutadditional DC isolation.

According to another embodiment, an amplifier is provided having anoutput switching stage, a low voltage stage operable to generate a drivesignal for driving the output switching stage, and isolation circuitryoperable to provide DC isolation between the low voltage stage and theoutput switching stage. Current sensing circuitry is operable to detectan overcurrent condition in the output switching stage and generate afault signal. The fault signal is characterized by a signal level whichis compatible with the low voltage stage without additional DCisolation.

A further understanding of the nature and advantages of the presentinvention may be realized by reference to the remaining portions of thespecification and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a specific embodiment of theinvention.

FIG. 2 is a schematic diagram of an alternative circuit for use with aspecific embodiment of the invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Reference will now be made in detail to specific embodiments of theinvention including the best modes contemplated by the inventor forcarrying out the invention. Examples of these specific embodiments areillustrated in the accompanying drawings. While the invention isdescribed in conjunction with these specific embodiments, it will beunderstood that it is not intended to limit the invention to thedescribed embodiments. On the contrary, it is intended to coveralternatives, modifications, and equivalents as may be included withinthe spirit and scope of the invention as defined by the appended claims.In the following description, specific details are set forth in order toprovide a thorough understanding of the present invention. The presentinvention may be practiced without some or all of these specificdetails. In addition, well known features may not have been described indetail to avoid unnecessarily obscuring the invention.

As mentioned above, a variety of techniques may be employed in switchingcircuit topologies to provide isolation between relatively high poweroutput devices and the relatively low power circuitry which drives them.Such techniques include capacitive and inductive coupling. Embodimentsof the present invention provide overcurrent sensing techniques whichmay be employed in conjunction with such isolation techniques.

FIG. 1 illustrates an exemplary switching amplifier in which aparticular embodiment of the invention may be implemented. It should benoted that the amplifier topology of FIG. 1 is shown and described toprovide an example of a context in which the present invention may bepracticed. However, the present invention is not limited to thedescribed context and may be practiced in any of a wide variety ofamplifier topologies in which DC isolation is provided.

Switching amplifier 100 includes a low voltage, noise-shaping signalprocessor 102 which generates complementary 1-bit digital signals Y and{overscore (Y)}. According to the embodiment shown, these 0˜5V signalsare level shifted by high current driver circuit 104 to 0˜10V signals toprovide the appropriate voltage and current levels to drive output FETs106 and 108.

Two capacitors C_(C) provide DC isolation between the output FETs andcurrent driver circuit 104 and, along with diodes D1 and D2, level shiftthe output signals of circuit 104 to VPP ˜VPP−10 volts at the gate ofhigh side FET 106, and VNN ˜VNN+10 volts at the gate of low side FET108. Where the FET driving voltages are relatively low, e.g., the 10volt range, diodes D1 and D2 may comprise signal diodes. Where the FETdriving voltages are higher, e.g., the 15 volt range, zener diodes maybe used to clamp the gates to ±10 volts.

Transistors Q1 and Q2 squelch the gates of output FETs 106 and 108 (inaccordance with the time constant corresponding to Rd and Cd) during thetime when supply voltages VPP and VNN are ramping up. That is, becauseof the RC time constants, the base terminals of transistors Q1 and Q2lag behind the supply voltages keeping transistors Q1 and Q2 turned on,effectively clamping the gates of the output FETs to the supplyvoltages, thereby ensuring that the output FETs remain off until thesupply voltages have stabilized and transistors Q1 and Q2 subsequentlyturn off. This eliminates potentially catastrophic results which mightotherwise occur at turn on as a result of the indeterminate state of thegates of the output FETs.

Significant benefits may be derived from the DC isolation provided bycapacitors Cc. For example, because driver circuit 104 is isolated fromthe high supply voltages (e.g., ±50V) required by the output FETs, itdoes not have the breakdown voltage requirements of a driver circuitdirectly coupled to the output switches. Thus, if driver circuit 104 isimplemented as an integrated circuit, a lower voltage (and thereforeless expensive) process may be employed in its fabrication.

In addition, the isolation of driver circuit 104 from the output FETscreates an opportunity to integrate driver circuit 104 with the lowvoltage circuitry of signal processor 102 in a single monolithicintegrated circuit (represented by dashed line 110). Thus, for example,in the exemplary embodiment shown in FIG. 1, a 16˜20V process withgeometries around 0.5 uM could be used for such a monolithic IC insteadof a 80˜100V process (with the correspondingly larger geometries) fordriver circuit 104, and a 5˜10V process for processor 102. In general,lower voltage processes are less expensive and more common than theirhigh voltage counterparts, allowing the designer to select from agreater variety of mature processes offered by a greater number ofsemiconductor fabs.

Overcurrent protection for amplifier 100 may be provided according to aspecific embodiment of the invention using a current sensing circuitwhich senses the current flowing in sense resistor Rs (which might, forexample, be in the range of 50–100 milliohms) and generates a faultsignal in response to an overcurrent condition. Op amp 120 is configuredwith resistors R₁ and R₂ to generate an output V_(O) equal toV₁−V₂+V_(R). Exemplary values for R₁ and R₂ are 200 kΩ and 6 kΩ,respectively. V_(R) may be selected to place the output of op amp 120 atthe desired level halfway between the +ref and −ref voltages associatedwith comparators 122 and 124, e.g., if −ref is zero volts and +ref is 5volts, V_(R) may be set at 2.5 volts.

If V_(O) goes outside of the range between −ref and +ref (i.e., a largevoltage appears across Rs), a fault signal is generated (e.g., at theoutput of OR gate 126) which may be used in any of a variety of ways tocontrol or shut down operation of amplifier 100. For example, the signalmay be used as an input to a “window” circuit, the output of which maybe used to turn off the output switches. One exemplary use of the faultsignal is described below with reference to FIG. 2. In view of the factthat the use of a fault indicator to control the operation of anamplifier is well within the knowledge of one of skill in the art,further details regarding the manner in which the fault signal generatedaccording to the invention may be used are not provided here.

The voltage levels associated with most of the current sensing circuitryare significantly lower than the voltages experienced by the sensingresistor Rs. Thus, the current sensing circuitry and any circuitryemploying the fault signal are effectively isolated from the highvoltages of the output stage of amplifier 100. Much of this circuitrycan therefore be implemented as relatively inexpensive, low voltagecircuitry as described above with reference to signal processor 102 anddriver circuitry 104. In fact, the current sensing circuitry (with theexception of the resistors connected directly to Rs) could beimplemented on a single die with either or both of circuits 102 and 104.

While the invention has been particularly shown and described withreference to specific embodiments thereof, it will be understood bythose skilled in the art that changes in the form and details of thedisclosed embodiments may be made without departing from the spirit orscope of the invention. For example, the present invention relatesgenerally to providing overcurrent protection in a switching amplifiertopology which employs some form of DC isolation. The techniquesdescribed herein may be applied to any of a wide variety of switchingcircuit topologies such as, for example, any type of digital or class Damplifier including sigma delta modulators, and modified sigma deltamodulators (e.g., Class T amplifiers available from Tripath TechnologyInc. of Santa Clara, Calif.). Examples of suitable amplifier topologiesare described in U.S. Pat. No. 5,777,512 for METHOD AND APPARATUS FOROVERSAMPLED, NOISE-SHAPING, MIXED-SIGNAL PROCESSING issued Jul. 7, 1998,the entire disclosure of which is incorporated herein by reference forall purposes. However, it will be understood that the invention is notlimited to the described topologies. For example, the present inventionmay also be applied to any type of pulse width modulation (PWM)amplifier, switch mode power supplies (SMPS), etc.

Moreover, the invention is not limited by the manner in which DCisolation is provided. That is, for example, the invention is notlimited to topologies employing capacitive coupling. Other DC isolationtechniques such as, for example, transformer or inductive coupling(e.g., transformers T1 and T2 of FIG. 1 in place of capacitors Cc), maybe employed with various embodiments of the invention.

In addition, instead of controlling the bases of Q1 and Q2 with the RCcircuits shown in FIG. 1, other types of control circuits may beemployed which include, but are not limited to, active circuit elementswhich are enabled for a programmable period of time or in response toone or more control signals. For example, according to an alternativeembodiment illustrated in FIG. 2, the base of Q1 is coupled to thecollector of another transistor Q3 which is driven to pull the base ofQ1 down for a period of time. The driving signal of such a transistorand the period of time could be controlled by any of a variety of analogor digital circuitry (represented by controller 202) including, forexample, a microprocessor or any other type of controller. Thiscircuitry may be incorporated or integrated with the driver circuitry orseparate therefrom. Such a controller may be configured to operateduring stabilization of the supply voltages (e.g., during power up).Such a controller may also be configured to control Q1 and Q2 duringamplifier operation independent of drive from the driver circuitry,e.g., in response to a fault condition such as, for example, anovercurrent condition indicated by the fault signal generated by OR gate126 of FIG. 1. In addition, and as will be understood, such a controllermay be configured to drive transistors Q1 and Q2 directly.

Finally, although various advantages, aspects, and objects of thepresent invention have been discussed herein with reference to variousembodiments, it will be understood that the scope of the inventionshould not be limited by reference to such advantages, aspects, andobjects. Rather, the scope of the invention should be determined withreference to the appended claims.

1. An amplifier, comprising: output switch circuitry comprisingcomplementary field effect transistors; a sense resistor operable totransmit an output current associated with the output switch circuitry;low-voltage circuitry operable to provide a drive signal to the outputswitch circuitry; isolation circuitry operable to provide DC isolationbetween the low-voltage circuitry and the output switch circuitry;current sensing circuitry directly connected to the sense resistor andoperable to sense the output current in the sense resistor and generatea fault signal in response to an overcurrent condition, the fault signalbeing characterized by a signal level which is compatible with thelow-voltage circuitry without additional DC isolation; and transientcontrol circuitry operable to keep the field effect transistors offduring stabilization of supply voltages associated with the field effecttransistors.
 2. The amplifier of claim 1 wherein the isolation circuitrycomprises one of a capacitor and a transfonner.
 3. The amplifier ofclaim 1 wherein the low-voltage circuitry comprises first circuitryoperable to generate complementary signals, and second circuitryoperable to level shift the complementary signals to signal levelsappropriate for driving the output switch circuitry.
 4. The amplifier ofclaim 3 wherein the first and second circuitry are integrated on asingle chip.
 5. The amplifier of claim 4 wherein the current sensingcircuitry is also integrated on the single chip.
 6. The amplifier ofclaim 1 wherein the transient control circuitry comprises clampingcircuits operable to clamp drive terminals of a corresponding one of thefield effect transistors to the associated supply voltage for apredetermined period of time.
 7. The amplifier of claim 6 wherein eachclamping circuit comprises a clamping switch coupled between thecorresponding supply voltage and drive terminal.
 8. The amplifier ofclaim 7 wherein the each clamping circuit further comprises an RCcircuit coupled to the corresponding supply voltage which is operable tosupply a gating signal to the clamping switch which lags behind thecorresponding supply voltage.
 9. A The amplifier of claim 8 wherein eachclamping circuit further comprises a second clamping switch which isoperable to provide a gating signal to the clamping switch.
 10. Theamplifier of claim 9 wherein each clamping circuit further comprises acontroller circuit which is operable to drive the second clampingswitch.
 11. The amplifier of claim 10 wherein the controller circuit isoperable to drive the second clamping switch in response to the faultsignal.
 12. The amplifier of claim 10 wherein the controller circuit isoperable to drive the second clamping switch during stabilization of thesupply voltage.
 13. The amplifier of claim 1 wherein the current sensingcircuitry is operable to convert a first sense voltage across the senseresistor to a second sense voltage for comparison to upper and lowerthreshold voltages, the fault signal being generated when the secondsense voltage does not fall between the upper and lower thresholdvoltages.
 14. The amplifier of claim 1 wherein the amplifier comprisesone of a digital amplifier, a pulse width modulator, and a switch modepower supply.
 15. The amplifier of claim 14 wherein the digitalamplifier comprises one of a sigma-delta modulator and a modifiedsigma-delta modulator.